1. Field of the Invention
With the increasing scarcity of quality wood lumber, it has become most desirable to seek alternative materials that can substitute for wood. Composite materials prepared from plastics and natural fibers are widely used as such wood substitutes. The use of these composite materials in articles of commerce is increasing rapidly. These composite materials can be used in a variety of products for structural and decorative architectural applications, as well as automotive uses. These composite materials may be cut, shaped, sanded, drilled and fastened in the same manner as natural wood.
Architectural products containing these composite materials prepared from plastics and natural fibers are frequently prepared by extrusion. The architectural products containing these composite materials can be used in both residential and commercial applications. Examples of such applications include decking, railing, fencing, posts, trim, moldings, siding, shingles, lattice, sills, and jambs. When compared to wood, consumers expect such composite materials to offer greater durability and weather resistance, with lower maintenance. Composite materials prepared from plastics and natural fibers are generally resistant to rot and insect attack.
Maximizing the durability of these composite materials prepared from plastics and natural fibers is of critical importance to consumers and producers alike. These composite materials are generally more costly than natural wood. Furthermore, they are also usually heavier than wood, which is a potential hardship to builders, and presents additional structural issues for architects and engineers. In order to overcome these drawbacks, composite materials should offer superior benefits in terms of durability and low maintenance.
Composite materials prepared from plastics and natural fibers may also be used for the production of articles in automotive applications. A variety of parts have been prepared such as knobs, interior door handles, and decorative interior fascia; as well as concealed parts such as sound-deadening panels, trunk liners, tire covers, bins, and carpet backings. Such parts are frequently pressed or molded.
Other uses for composite materials prepared from plastics and natural fibers include sheeting substitutes for fiberboard, particleboard and plywood. Composite materials can be used to make miscellaneous small articles such as planters, tubs, pots, and picture frames.
Useful composite materials can be prepared from various plastics, including polyethylene polypropylene and co-polyethylene-vinyl acetate. Recycled plastics are frequently used in order to lower cost. High-density polyethylene (HDPE) is particularly well-suited for use in decking, railing, fencing, and similar applications.
Natural fibers are used in the composite materials in order to reduce cost and weight of the composite material, and to improve the physical properties, particularly stiffness and tensile strength. Natural fibers are superior to synthetic fibers like carbon and glass as the natural fibers are of lower cost. Natural fibers are preferred over mineral fillers such as talc and mica as the natural fibers produce lighter composites. The natural fibers are usually waste products from other processes. Almost any cellulosic particulate can be used, including wood, newspaper, cardboard, straw, agricultural and plant residues, and the like.
A well-known problem in the formation of composite materials prepared from plastics and natural fibers is the incompatibility of the fiber with the plastic. Natural fibers are hydrophilic, with many free polar hydroxyl groups on the surface. Plastics are hydrophobic. Therefore, the plastics do not readily wet the surface of the natural fiber and adhere thereto. This causes a loss of strength in the resulting composite material.
This problem can be overcome by the addition of coupling agents to the composite material. Coupling agents are thought to function by the reaction of a reactive anhydride or acid moiety with hydroxyl groups on the surface of the fiber to form an ester linkage. The hydrophobic polymer chains extend outwards from the fiber surface, where they can interact with the bulk of the polymer matrix. The exact nature of the interaction will depend upon the choice of coupling agent and polymer, and the extent of crystallinity of the polymer. The coupling agent generally serves as a transitional bridge that improves the adhesion of the plastic to the natural fiber surface. Improved adhesion can result in improved physical properties for the composite material prepared from plastics and natural fibers, particularly the tensile and flexural strength, resistance to water absorption and creep, and reduction in the linear coefficient of thermal expansion (LCTE).
If the composite material prepared from plastics and natural fibers material is to be processed by extrusion, a lubricant is normally added to aid in passage of the composite material from the die. An improperly lubricated system will cause the composite material to extrude at differing rates. This can result in various unacceptable physical defects in the composite material, ranging from a scaly sharkskin appearance to saw-toothed edge tears.
A commonly used lubricant system is a blend of zinc stearate with an N,N′-ethylene bis-stearamide (EBS) wax. Other lubricants include calcium stearate, magnesium stearate, non-metallic stearates; paraffin wax, polyester wax, polypropylene wax, fatty acid derived bis-amides, ethylene bis-oleamide, esters such as stearyl stearate, distearyl phthalate, pentaerythritol adipate stearate, ethylene glycol distearate, pentaerythritol tetrastearate, glycerol tristearate, polyethylene glycol 400 monostearate, glycerol monooleate, glycerol distearate, and blended complex modified fatty acid esters.
Unfortunately, it has been found that the commonly used zinc stearate-EBS lubricant system interferes with the coupling agent. It has been suggested that the zinc stearate is responsible for the interference with the maleic anhydride grafted olefin coupling agents. The stearate ring opens and esterifies the anhydride group, while the zinc complexes the resulting carboxylate. The coupling agent then provides significantly less improvement to the physical properties of the composite material prepared from plastics and natural fibers than expected.
Some commercial composite materials prepared from plastics and natural fibers use a zinc stearate/ethylene bis-stearamide lubricant system without any coupling agent. These formulations extrude quite readily, and provide product of excellent appearance. The physical properties of these composites are deficient when compared to those of natural wood.
2. Description of Related Art
U.S. Pat. No. 6,632,863 claims a two-step process whereby wood-plastic composite (WPC) pellets are made which may contain from 0 to 35% of an additive selected from a group of inorganic fillers and lubricants. The lubricants are subsequently selected from the group of zinc stearate and ethylene bis-stearamide. The inorganic fillers are subsequently selected from the group of talc and mica. The WPC pellet is subsequently re-extruded with additional plastic and additives.
U.S. Pat. Nos. 6,498,205 and 6,344,504 claim a WPC prepared from a powder blend of wood fiber/flour, plastic, and a lubricant selected from zinc stearate and a wax. The specifications give ethylene bis-stearamide as an example of a wax.
U.S. Pat. Nos. 6,682,789 and 6,265,037 and U.S. patent application Ser. No. 2001/0051243 teach a polymer/fiber composite composition, a process for WPC manufacture, and disclose an extruded profile thereby prepared, based on polyolefins prepared by modern metallocene catalysis with a distribution ratio (Mw/Mn) of 3 to 6, a melt index of less than 2 gm/10 min, where moisture content is less than 5000 parts per million (ppm) of the formulation, and the fiber dimensions are from 100 to 2000 micrometers (μm), with an aspect ratio of 1:2 to about 1:5. Maleic anhydride grafted olefin coupling agents are used, and the composition may contain lubricants and inorganic fillers.
U.S. patent application Ser. No. 2003/0229160 teaches that ethylene bis-amides made from carboxylic acids of 6 to 10 carbons are superior lubricants to EBS/zinc stearate blends presently used in wood. This application further teaches a composite comprising about 30 to about 70 weight percent polymer, about 70 to about 30 weight percent agricultural waste fiber, and about 1 to 7 weight percent of a lubricant consisting of an amide, an alkylene bisamide, or a combination thereof.
U.S. Pat. No. 4,791,020 teaches the use of up to 40% inorganic filler in a WPC, where the filler is selected from the group consisting of mica, talc, calcium carbonate, silica, glass fiber, asbestos and wollastonite.
U.S. Pat. No. 5,326,513 teaches a process for producing plastic fiber boards using a material selected from balls of expanded glass, expanded clay, pumice granules and mica, admixed with a foaming and hardenable organic and inorganic binder, where the organic binder is an epoxy resin or a polyurethane phenol resin.
U.S. Pat. No. 5,516,472 describes a process (“Strandex Process”) widely licensed and used in the WPC industry. The patent teaches that the preferred formulation to be used with the above process includes 3.0 parts zinc stearate and 2.0 parts external (paraffin-type) wax per hundred parts wood flour.